Generally, this invention relates to the preparation of photographic plates for use in planographic printing as well as methods of making the same. More specifically, new and improved intermediate coatings have been developed which may be applied to preconditioned support members by means of a single step bath process prior to the application of a light-sensitive coating.
Intermediate coatings are ordinarily used on lithographic plates to improve bonding or anchoring of light-sensitive materials to support plates and to inhibit any deleterious reaction between the outer photo-sensitive coating and the support member. Besides performing these necessary functions, it has been discovered that the intermediate coatings of the instant invention comprising the reaction product of a novel and highly branched, highly stable polyalkylenimine-urea-aldehyde resin and a polyacrylic resin applied either to a grained or ungrained, anodized or unanodized metallic substrate produces an extremely durable surface which in turn gives a longer running lithographic plate. In addition to their longer running characteristics, the plates of the instant invention may be further distinquished by their substantially extended shelf-life which allows storage for a prolonged period of time prior to use. Such features stand out especially when the coatings are applied to anodized aluminum support members.
The planographic printing plates described herein are not limited solely to the more popular presensitized type plates, but also include the so called wipe-on plates. Under the present invention, a wipe-on plate will consist of the above-mentioned top-coating with the exclusion of the light sensitive resin. In such case, the plate maker applies the light sensitive coating prior to use. On the other hand, a presensitized plate includes the photosensitive coating. In either case, plates of the present invention may be treated in the conventional manner. For example, a plate may be exposed to a carbon arc or a pulsed xenon light source for the appropriate exposure period. The plate can then be developed with one or two step developing lacquers by hand techniques or by developing machines.
Improved planographic printing plates are well known in the graphic arts industry. One such plate is commonly prepared by coating an aluminum sheet with an aqueous solution of an alkali metal silicate. This plate is usually dried and a light sensitive coating, such as a diazo resin is then applied. Upon exposure to light through a stencil or negative, the exposed diazo will form a light-hardened water insoluble hydrophobic surface. The unexposed surfaces may then be removed by an aqueous desensitizer. This plate is described in U.S. Pat. No. 2,714,066 to Jewett et al. Plates of this type, however, unlike those of the present invention are characterized by markedly shorter press runs and diminished shelf-life.
It is also known in the art that anodizing aluminum support members improves corrosion resistance and surface hardness of printing plates. An anodized aluminum printing plate has been described in U.S. Pat. No. 3,181,461 to Fromson. Plates disclosed by Fromson, although exhibiting longer press run qualities than unanodized plates as in Jewett et al. nevertheless fail to provide the unexpected high press runs experienced with preconditioned plates having the intermediate coating as disclosed herein.
The remarkable printing qualities and stability of the plates of the present invention are attributable to the novel polyalkylenimine-urea aldehyde resins employed therein. The abundance of tertiary amino groups in the resin provide a high degree of compatibility with other water soluble resins, including polyacrylic resins. In this regard, reaction between the polyacrylic resin and imine resin may be characterized whereby available carboxy groups of the acrylic compound react with the tertiary amino groups of the imine resin to form salts. In contradistinction, other plates have also employed polyacids such as polyacrylic acid, however, they were used alone as intermediate coatings in which case the only probable reaction would take place between the metallic surface and the carboxylic groups of acid. The surface was rendered somewhat hydrophilic, but polyacids alone failed to lengthen the useful life on the plate appreciably.
Still other plates have used monomeric organic acids such as acrylic acid, methacrylic acid and their water soluble salts in conjunction with conventional amino resins such as urea and melamine formaldehyde resins. In this case, two bath stations are required in the coating process wherein the amino resin would first be applied to the support plate and then the monomeric acid applied by means of a second bath. An in-situ reaction would occur on the plate whereby the amino resin layer and the acid layer react, the former acting at the ethylenic linkage of the acid. This type of plate has been less than satisfactory, not just because of toxicity, odors and added production costs resulting from additional coating steps, but also by reason of its relatively brief shelf-life and storage properties as well as possible scumming tendencies. The abbreviated shelf-life of such plate may be attributed to the use of conventional, less stable amino resins as intermediate coatings.
Accordingly, it is a principal object of this invention to provide a series of novel polymeric compositions for coating preconditioned metallic support plates.
Another principal object of the present invention is to provide a method of use whereby polymeric compositions are applied to metallic plates as top-coat layers by a single-bath process.
It is still another principal object to provide an unusually durable, long running planographic plate comprising a preconditioned support member coated with an intermediate top-coat or subbase layer comprising novel polymeric compositions.
It is a further object to provide presensitized and wipe-on photolithographic plates having substantially improved wearing qualities comprising an anodized aluminum plate, an intermediate coating, and a final coating of a photo-sensitive material.
A still further object of the present invention is to provide a photolithographic plate which has a greatly extended shelf-life.
These and other objects, features and advantages of this invention will become apparent to those skilled in the art after a reading of the following more detailed description.